In attempting to identify pith-casts the student must keep in view the probable differences presented by the branching rhizome, the main aerial branches and the finer shoots of the same individual. The long internodal ridges of some casts may be mistaken for the parallel veins of such a leaf as _Cordaites_, a Palaeozoic Gymnosperm, if there are no nodes visible on the specimen. The fossil figured by Lindley and Hutton[755] as _Poacites_, and regarded by them as a Monocotyledon, is no doubt a portion of a Calamite with very long internodes. An interesting example of incorrect determination has recently been pointed out by Nathorst[756] in the case of certain casts from Bear Island, originally described by Heer as examples of _Calamites_; the vertical rows of leaf-trace casts on a _Knorria_ were mistaken for the ribs of a Calamite stem. The specimens in the Stockholm Museum fully bear out Nathorst’s interpretation. The undulating course of internodal ridges and grooves is not in itself a character of specific value. If a Calamite stem were bent slightly, the wood and medullary-ray tissues on the concave side might adapt themselves to the shortening of the stem by becoming more or less folded, and a cast of such a stem would show undulating ridges and grooves on one side and straight ones on the other[757]. A convenient classification of Calamite casts was proposed by Weiss in 1884, founded chiefly on the number and manner of occurrence of branch-scars—or rather branch-depressions—on the surface of pith-casts. Weiss[758] recognised the imperfection of his proposed grouping, and Zeiller[759] has also expressed reasonable doubts as to the scientific value of such group-characters. Weiss instituted three subgenera—_Calamitina_, _Eucalamites_ and _Stylocalamites_, which are made use of as convenient terms in descriptive treatment of Calamite casts. The following account of a few of the more typical casts may serve to illustrate the methods employed in the description of such specimens; the synonomy given for the different species is not intended to be complete, but it is added with a view to drawing attention to the necessity for careful comparison in systematic work. A. _Calamitina._ [Illustration: FIG. 99. _Calamites_ (_Calamitina_) _Göpperti_ (Ett.). _b_, branch scars. From a specimen in the Manchester Museum, Owens College. ¼ nat. size.]